57 MICRO-SPECTROSCOPE AND POLAR/SCOPE. 



accurately focused on the slit. The eye-lens is movable, and when the 

 prism is swung aside it is very easy to focus the slit as one focused for 

 the ocular micrometer (§ 114). If one now uses daylight there will be 

 seen in the spectrum the dark Fraunhofer lines (PL V, Fig. 45, E. F., 

 etc.). 



To show the necessity of focusing the slit, move the eye-lens down 

 or up as far as possible, and the Fraunhofer lines cannot be seen. 

 While looking into the spectroscope move the ocular lens up or down 

 and when it is focused the Fraunhofer lines will reappear. As the 

 different colors of the spectrum have different wave lengths, it is neces- 

 sary to focus the slit for each color if the sharpest possible pictures are 

 desired. 



§ 137. Comparison or Double Spectrum. — In order to compare 

 the spectra of two different substances it is desirable to be able 

 to examine their spectra side by side. This is provided for in the 

 the better forms of micro-spectroscopes by a prism just below the slit, 

 so placed that light entering it from a mirror at the side of the drum 

 shall be totally reflected in a vertical direction, and thus parallel with 

 the rays from the microscope. The two spectra will be side by side 

 with a narrow dark line separating them. If now the slit is well fo- 

 cused and daylight be sent through the microscope and into the side to 

 the reflecting or comparison prism, the colored bands and the Fraun- 

 hofer dark lines will appear directly continuous across the two spectra. 

 The prism for the comparison spectrum is movable and may be entirely 

 thrown out of the field if desired. When it is to be used, it is thrown 

 about half way across the field so that the two spectrums shall have 

 about the same width. 



§ 138. Scale of Wave Lengths. — In the Abbe micro-spectroscope 

 the scale is in a separate tube near the top of the prism and at right angles 

 to the prism- tube. A special mirror serves to light the scale, which is 

 projected upon the spectrum by a lens in the scale-tube. This scale is 

 of the Angstrom form, and the wave lengths of any part of the spec- 

 trum may be read off directly, after the scale is once set in the proper 

 position, that is, when it is set so that any given wave length on the 

 scale is opposite the part of the spectrum known by previous investiga- 

 tion to have that particular wave length. The point most often selected 

 for setting the scale is opposite the sodium lines where the wave length 

 is, according to Angstrom, 0.5892 /a. In adjusting the scale, one may 

 focus very sharply the dark sodium line of the solar spectrum and set 

 the scale so that the number 0.589 is opposite the sodium or D line, or 

 a method that is frequently used and serves to illustrate § 129 C, is to 

 sprinkle some salt of sodium (carbonate of sodium is good) in an alco- 

 hol lamp flame and to examine this flame. If this is done in a dark- 



